Concept 3 Review

Meselson and Stahl's Experiment Proving the Semiconservative Model of DNA Replication

Meselson and Stahl proved that the semiconservative model of DNA replication was correct.

The following figures illustrate the Meselson and Stahl experiment which demonstrated the correctness of the semiconservative model of DNA replication. You will be able to "run" the CsCl (cesium chloride) centrifugations yourself and see the DNA band or bands for each culture condition (right tube in each case), compared with the control band for 14N-labeled (light) DNA (left tube). Click on each DNA sample tube to run the centrifugation. Click on "Closer Look" for more information on Cesium Chloride.

  1. Bacteria grown in 14N or 15N medium.
  2. Samples removed and DNA extracted.
  3. Spin DNA samples in CaCl by rolling your mouse over each DNA sample tube below.

The Meselson and Stahl Experiment Described

Meselson and Stahl tested the hypothesis of DNA replication. They cultured bacteria in a 15N medium. 15N is a heavy isotope of nitrogen so the DNA synthesized is of heavy density. They then shifted the bacteria to a 14N medium, DNA was isolated at different times corresponding to replication cycles 0, 1, and 2. After one replication cycle, the DNA was all of intermediate density. This rules out the conservative replication model, which predicts that both heavy density DNA and light density DNA will be present, but none of intermediate density will be present. This result is consistent with the semiconservative replication model, which predicts that all DNA molecules will consist of one 15N-labeled DNA strand and one 14N-labeled DNA strand. The result does not rule out the dispersive replication model, which also predicts that all DNA will be of intermediate density, consisting of interspersed double-stranded 15N-labeled and 14N-labeled segments.

After two replication cycles, two bands of DNA were seen, one of intermediate density and one of light density. This result is exactly what the semiconservative model predicts: half should be 15N-14N intermediate density DNA and half should be 14N-14N light density DNA. This result rules out the dispersive replication model, which predicts that after replication cycle 1, the DNA density of all DNA molecules will gradually become lower, so no intermediate density DNA should remain at replication cycle 2. The semiconservative model is correct.